FIG. 1 shows a geostationary communication satellite system 100 comprising a plurality of satellites 1021 to 102i orbiting the earth 104. Satellite 1021 is separated from adjacent satellites 1022 and 1023 by approximately a 2° arc (the arc is shown by the separation between the dashed lines on each of FIGS. 1, 2, 5, and 6, and is typical for geostationary satellites in the United States). Earth 104 has a plurality of earth stations 1061 to 106n. Each earth station 106 includes a satellite transmitting and receiving antenna 108. Communication system 100 operates when antenna 108 generates a communication signal 110 that is received by, for example, satellite 1021, and visa versa.
As communication signal 110 travels from, for example, earth station 1061 to its intended destination at satellite 1021 it spreads over an area 112. If communication signal 110 spreads beyond the 2° arc between satellite 1021 and the adjacent satellites 1022 and 1023, then all three satellites 1021, 1022, and 1023 would process communication signal 110 as if it was intended for them. One reason this occurs is that communication signal 110 does not experience significant signal attenuation at the edge of area 112. In order to prevent satellites 1022 and 1023 from processing communication signal 110, antenna 108 generates a narrow beam communication signal, instead of a wide beam communication signal.
The most widely used radio frequency bands for satellite communication are the Ku- and C-bands. In both of these bands, a conventional parabolic reflector antenna generates a narrow communication signal to prevent adjacent satellites from processing communication signals not intended for them. The parabolic reflector antenna for the Ku-band may have a relatively small diameter. The small parabolic reflector antenna provides an efficient, cost-effective mechanism for allowing an earth station to communicate with an individual satellite. Unfortunately, Ku-band radio signals attenuate in atmospheric conditions consistent with periods of moderate-to-heavy precipitation, i.e., rain, sleet, or snow. In most cases, providing facilities with sufficient power to compensate for severe signal attenuation is uneconomical. As a result, satellite communications systems operating in the Ku-band experience periodic system outages that are unacceptable for time critical applications.
To avoid periodic system outages due to atmospheric conditions, earth stations typically transmit and receive data using C-band radio frequencies. These frequencies are much less susceptible to attenuation due to precipitation. Therefore, C-band transmitters can economically provide sufficient signal margin to overcome any signal attenuation due to atmospheric conditions. Unfortunately, to generate narrow communication signal beams, C-band parabolic antennas need to be larger than Ku-band antennas. In fact, the minimum C-band parabolic antenna diameter that prevents communication signal 110 from interfering with satellites 1022 or 1023 (See FIG. 1) is approximately 3.7 meters. For many applications, however, the installation of a 3.7 meter diameter antenna is too unwieldy, aesthetically unseemly, and/or not structurally prudent. Therefore, it would be desirable to use smaller diameter parabolic reflective antenna to transmit C-band radio frequencies while avoiding unnecessary interference with adjacent satellites.
Further, during short periods of each day for several days immediately before and after the vernal and autumnal equinoxes, the sun transits behind geostationary satellites as seen from an earth station's receiving antenna (i.e., from the perspective of the earth station, the sun passes behind the geostationary satellite). The sun emits a great deal of energy in the form of electromagnetic radiation in the bandwidth occupied by radio wave communications. Therefore, when the sun is located within the beamwidth of the receiving antenna, its energy causes interference in the form of radio frequency noise. This noise causes a decrease in the signal-to-noise ratio of the earth station's receiver, and can render the earth station inoperative until the sun completes its transit of the antenna's beamwidth.
Because the relative movement of the earth with respect to the sun is known to a high degree of precision, satellite communication system operators are forewarned of the time when the sun will transit the beamwidth of a receiving antenna. Knowledge of a pending problem, however, is only useful if the system operators can keep the system operational during these periods.
For conventional satellite systems, each individual receive antenna might be effected by the sun's positioning during this period. Some conventional systems use costly terrestrial communications facilities to provide continuing operations as the sun transits behind a satellite with respect to its earth station's receiving antenna. Other systems remain off-the-air for the duration of these periods. The inherent inconvenience of this option, however, renders it particularly unattractive. Finally, some conventional satellite systems continue operation by switching each earth station's antenna to a secondary satellite during the period that the sun is within the beamwidth of the antenna. This process requires manual intervention and/or complex automated mechanical mechanisms to perform the daily repositioning of the antenna during its sun transit outage. The cost of the daily repositioning of each antenna so effected renders this option uneconomical.
Therefore, a need exists for a satellite communication system to efficiently provide communication during sun transit outages.